Jon Chorover

The chemistry of pedogenic thresholds

Abstract Pedogenesis can be slow or fast depending on the internal chemical response to environmental forcing factors. When a shift in the external environment does not produce any pedogenic change even though one is expected, the soil is said to be in a state of pedogenic inertia. In contrast, soil properties sometimes change suddenly and irreversibly in a threshold response to external stimuli or internal change in soil processes. Significant progress has been made in understanding the thermodynamics and kinetics of soil-property change. Even in the open soil system, the direction of change can be determined from measures of disequilibrium. Favorable reactions may proceed in parallel, but the most prevalent and rapid ones have the greatest impact on product formation. Simultaneous acid–base, ion exchange, redox and mineral-transformation reactions interact to determine the direction and rate of change. The nature of the governing reactions is such that soils are well buffered to pH change in the alkaline and strongly acid regions but far less so in the neutral to slightly acid zones. Organic matter inputs may drive oxidation–reduction processes through a stepwise consumption of electron acceptors (thereby producing thresholds) but disequilibrium among redox couples and regeneration of redox buffer capacity may attenuate this response. Synthesis of secondary minerals, ranging from carbonates and smectites to kaolinite and oxides, forms a basis for many of the reported cases of pedogenic inertia and thresholds. Mineralogical change tends to occur in a serial, irreversible fashion that, under favorable environmental conditions, can lead to large accumulations of specific minerals whose crystallinity evolves over time. These accumulations and associated “ripening” processes can channel soil processes along existing pathways or they can force thresholds by causing changes in water flux and kinetic pathways.

Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces

Experiments were conducted to compare the affinity and reactivity of three different minerals for natural organic matter (NOM) in forest floor leachate (FFL) from hardwood and pine forests. The FFLs were acidic (pH 4) with ionic strengths of 1.4 mM (hardwood) and 1.1 mM (pine), and they contained larger organic molecules (weight average molecular weights (Mw) 5 5- 6 kDa) than has been reported recently for surface waters using similar methods. A synthetic diluent solution was prepared to match the inorganic chemistry of the FFL and to provide a range of initial dissolved organic carbon (DOC) concentrations (0 -14 0gCm 23 ) for reaction with goethite (a-FeOOH), birnessite (d-MnO2) and smectite (montmorillonite, SWy-2) in suspension, and in corresponding blanks. A variety of macroscopic and spectroscopic methods were employed to show that reaction with the three minerals resulted in distinctly different NOM adsorption, fractionation and transformation patterns. Goethite exhibited a steep initial slope in the adsorption isotherm and a maximum retention of 10. 5gCk g 21 . The isotherm for montmorillonite was more linear, but equal amounts of C were adsorbed to goethite and montmorillonite (per unit sorbent mass) at maximum DOC. Whereas preferential uptake of high Mw, aromatic constituents via ligand exchange was observed for goethite, compounds of lower than average Mw were retained on montmorillonite and no preference for aromatic moieties was observed. Birnessite, which has an isoelectric point of pH , 2, retained low amounts of organic C (, 2gCk g 21 ) but exhibited the highest propensity for oxidative transformation of the NOM. The data indicate that fractionation behavior of NOM is dependent on mineral surface chemistry in addition to sorbent affinity for organic C. This work also emphasizes the fact that abiotic transformation reactions must be considered in studies of NOM interaction with Fe(III) and Mn(IV) containing solid phases. Copyright

Rapid abiotic transformation of nitrate in an acid forest soil

Nitrate immobilization into organic matter isthought to require catalysis by the enzymes ofsoil microorganisms. However, recent studiessuggest that nitrate added to soil isimmobilized rapidly and this process mayinclude abiotic pathways. We amended living andsterilized soil with 15N-labeled nitrateand nitrite to investigate biotic and abioticimmobilization. We report rapid transformationof nitrate in incubations of the O layer offorest soils that have been sterilized toprevent microbial activity and to denaturemicrobial enzymes. Approximately 30, 40, and60% of the 15N-labeled nitrate added tolive, irradiated, or autoclaved organic horizonsoil disappeared from the extractableinorganic-N pool in less than 15 minutes. About5% or less of the nitrate was recovered asinsoluble organic N in live and sterilizedsoil, and the remainder was determined to besoluble organic N. Added 15N-nitrite,however, was either lost to gaseous N orincorporated into an insoluble organic N formin both live and sterile organic soils. Hence,the fate and pathway of apparent abioticnitrate immobilization differs from thebetter-known mechanisms of nitrite reactionswith soil organic matter. Nitrate and nitriteadded to live A-horizon soil was largelyrecovered in the form added, suggesting thatrapid conversion of nitrate to solubleorganic-N may be limited to C-rich organichorizons. The processes by which this temperateforest soil transforms added nitrate to solubleorganic-N cannot be explained by establishedmechanisms, but appears to be due to abioticprocesses in the organic horizon.

Tree Species Effects on Soil Organic Matter Dynamics: The Role of Soil Cation Composition

A bstractWe studied the influence of tree species on soil carbon and nitrogen (N) dynamics in a common garden of replicated monocultures of fourteen angiosperm and gymnosperm, broadleaf and needleleaf species in southwestern Poland. We hypothesized that species would influence soil organic matter (SOM) decomposition primarily via effects on biogeochemical recalcitrance, with species having tissues with high lignin concentrations retarding rates of decomposition in the O and A horizons. Additionally, because prior work demonstrated substantial divergence in foliar and soil base cation concentrations and soil pH among species, we hypothesized that species would influence chemical stabilization of SOM via cation bridging to mineral surfaces in the A-horizon. Our hypotheses were only partially supported: SOM decomposition and microbial biomass were unrelated to plant tissue lignin concentrations, but in the mineral horizon, were significantly negatively related to the percentage of the cation exchange complex (CEC) occupied by polyvalent acidic (hydrolyzing) cations (Al and Fe), likely because these cations stabilize SOM via cation bridging and flocculation and/or because of inhibitory effects of Al or low pH on decomposers. Percent CEC occupied by exchangeable Al and Fe was in turn related to both soil clay content (a parent material characteristic) and root Ca concentrations (a species characteristic). In contrast, species influenced soil N dynamics largely via variation in tissue N concentration. In both laboratory and in situ assays, species having high-N roots exhibited faster rates of net N mineralization and nitrification. Nitrification:mineralization ratios were greater, though, under species with high exchangeable soil Ca2+. Our results indicate that tree species contribute to variation in SOM dynamics, even in the mineral soil horizons. To our knowledge the influence of tree species on SOM decomposition via cation biogeochemistry has not been demonstrated previously, but could be important in other poorly buffered systems dominated by tree species that differ in cation nutrition or that are influenced by acidic deposition.

Transport and fractionation of dissolved organic matter in soil columns

Dissolved organic matter (DOM) is a heterogeneous mixture of organic compounds that plays an important role in the movement of DOM-associated pollutants. In this study, transport and fractionation of DOM in soils was investigated in flow-through soil columns. Dissolved organic matter derived from spent mushroom substrate weathering was pumped through packed columns (2.5 cm × 10 cm) comprising a coarse-loamy subsoil (mixed, semiactive, mesic Typic Hapludult), and effluents were monitored for changes in the composition of DOM. Effluent DOM was characterized for UV absorbance, molecular weight, acidity, and hydrophilicity. Transport through the columns resulted in preferential retention of specific DOM constituents as indicated by comparison with a Br− tracer. During the transport process, effluent DOM exhibited decreasing values of E2/E3 (from 10.3 to 6.2), acidity (from 20.8 to 13.1 mmolc g−1 C), and hydrophilicity (39.0 to 28.4%), and increasing values of molar absorptivity (from 164 to 310 L mol−1C cm−1) and number and weight-averaged molecular weight (from 1770 to 3150 and 2450 to 4180 Da, respectively). These results indicate that DOM fractions with higher molecular weight, higher molar absorptivity, lower E2/E3 ratio, lower acidity, and lower hydrophilicity were adsorbed preferentially by soil minerals, whereas the inverse fractions were transported preferentially. The adsorbed DOM could not be completely desorbed by DOM-free background solution, indicating a strongly bound fraction. Sorptive fractionation of DOM during transport likely affects the transport behavior of DOM-complexed constituents.

Surface charge characteristics of kaolinitic tropical soils

Surface charge properties of four representative kaolinitic soils from the humid tropics (Brazil) were investigated by a methodology involving independent measurements of net total permanent and variable charge components. Permanent structural charge was determined by Cs+ adsorption, whereas variable charge was quantified by simultaneous proton titration and background electrolyte (LiCl) adsorption measurements. Data were obtained for homoionic soils suspended in LiCl solutions of ionic strength 1–10 mmol L−1 and pH value 2–6. Corrections were made in the titration data for proton consumption resulting from dissolution and aqueous-phase complexation reactions. Conjunctive use of proton titration and electrolyte adsorption data yielded independent assessments of proton surface charge densities and points of zero charge. The surface charge data were tested successfully for consistency with the law of surface charge balance. Three of the soils exhibited similar surface charge behavior, with no pronounced effect of differences in either organic C or Fe and aluminum oxide content. One soil containing significant manganese oxides showed points of zero charge well below those of the other three soils. The point of zero net charge (p.z.n.c) for the soils was ≤4, lower than values reported for specimen kaolinite. The point of zero net proton charge (p.z.n.p.c.) increased with decreasing ionic strength. In all cases, the presence of small quantities of structural charge in 2:1 clay minerals had a significant effect on surface charge properties; e.g., for all soils, p.z.n.c. < p.z.n.p.c. These characteristics of surface charge were shown to be consistent with the behavior of a mixture of kaolinite, organic matter, and a small quantity of 2:1 clay minerals. In conformity with the law of surface charge balance, ionic strength effects were found to be removed by plotting net adsorbed ion charge against net proton surface charge density.

Solution chemistry profiles of mixed-conifer forests before and after fire

Solution chemistry profiles of mixed-conifer forests in granitic catchments of the Sierra Nevada were measured for three years before (1987–1990) and three years after (1990–1993) prescribed fire. Wet deposition, throughfall and soil solution samplers were installed in both white-fir and giant-sequoia dominated forest stands underlain by poorly developed inceptisols. Stream water chemistry was monitored as part of an ongoing study of catchment outputs. Calcium, NO3− and Cl− were the major ions in precipitation. Canopy leaching increased mean concentrations of all major ions, especially K+ and Ca2+. Water flux through the soil occurred largely during spring snowmelt. Forest floor leachate represented the most concentrated solutions of major ions. Interaction with the mineral soil decreased mean concentrations of most species and the average composition of soil solutions closely resembled stream water at baseflow. Bicarbonate alkalinity, Ca2+, Mg2+, and Na+ were enriched in stream water relative to precipitation whereas inputs of H+, NH4+, NO3− and SO42− were retained within the catchments.Burning of the forest understory and litter layer increased solute concentrations in soil solution and stream water. Mean soil solution Ca2+, Mg2+ and K+ concentrations increased more than 10 fold, but the relative predominance of these cations was not affected by burning. Sulfate concentration, which was very low in soil solutions of undisturbed stands (<25 mmolc m−3), increased more than 100 times following fire. Ammonium concentration exhibited a rapid, short-term increase and then a decrease below pre-burn levels. Changes in soil solution chemistry were reflected in catchment outputs.

FTIR spectroscopic study of biogenic Mn-oxide formation by Pseudomonas putida GB-1

Biomineralization in heterogeneous aqueous systems results from a complex association between pre-existing surfaces, bacterial cells, extracellular biomacromolecules, and neoformed precipitates. Fourier transform infrared (FTIR) spectroscopy was used in several complementary sample introduction modes (attenuated total reflectance [ATR], diffuse reflectance [DRIFT], and transmission) to investigate the processes of cell adhesion, biofilm growth, and biological Mn-oxidation by Pseudomonas putida strain GB-1. Distinct differences in the adhesive properties of GB-1 were observed upon Mn oxidation. No adhesion to the ZnSe crystal surface was observed for planktonic GB-1 cells coated with biogenic MnO x , whereas cell adhesion was extensive and a GB-1 biofilm was readily grown on ZnSe, CdTe, and Ge crystals prior to Mn-oxidation. IR peak intensity ratios reveal changes in biomolecular (carbohydrate, phosphate, and protein) composition during biologically catalyzed Mn-oxidation. In situ monitoring via ATR-FTIR of an active GB-1 biofilm and DRIFT data revealed an increase in extracellular protein (amide I and II) during Mn(II) oxidation, whereas transmission mode measurements suggest an overall increase in carbohydrate and phosphate moieties. The FTIR spectrum of biogenic Mn oxide comprises Mn-O stretching vibrations characteristic of various known Mn oxides (e.g., “acid” birnessite, romanechite, todorokite), but it is not identical to known synthetic solids, possibly because of solid-phase incorporation of biomolecular constituents. The results suggest that, when biogenic MnO x accumulates on the surfaces of planktonic cells, adhesion of the bacteria to other negatively charged surfaces is hindered via blocking of surficial proteins.

Hydrological Partitioning in the Critical Zone: Recent Advances and Opportunities for Developing Transferable Understanding of Water Cycle Dynamics

Hydrology is an integrative discipline linking the broad array of water-related research with physical, ecological, and social sciences. The increasing breadth of hydrological research, often where subdisciplines of hydrology partner with related sciences, reflects the central importance of water to environmental science, while highlighting the fractured nature of the discipline itself. This lack of coordination among hydrologic subdisciplines has hindered the development of hydrologic theory and integrated models capable of predicting hydrologic partitioning across time and space. The recent development of the concept of the critical zone (CZ), an open system extending from the top of the canopy to the base of groundwater, brings together multiple hydrological subdisciplines with related physical and ecological sciences. Observations obtained by CZ researchers provide a diverse range of complementary process and structural data to evaluate both conceptual and numerical models. Consequently, a cross-site focus on “critical zone hydrology” has potential to advance the discipline of hydrology and to facilitate the transition of CZ observatories into a research network with immediate societal relevance. Here we review recent work in catchment hydrology and hydrochemistry, hydrogeology, and ecohydrology that highlights a common knowledge gap in how precipitation is partitioned in the critical zone: “how is the amount, routing, and residence time of water in the subsurface related to the biogeophysical structure of the CZ?” Addressing this question will require coordination among hydrologic subdisciplines and interfacing sciences, and catalyze rapid progress in understanding current CZ structure and predicting how climate and land cover changes will affect hydrologic partitioning. This article is protected by copyright. All rights reserved.

Effect of arbuscular mycorrhizal fungi on plant biomass and the rhizosphere microbial community structure of mesquite grown in acidic lead/zinc mine tailings

Mine tailings in arid and semi-arid environments are barren of vegetation and subject to eolian dispersion and water erosion. Revegetation is a cost-effective strategy to reduce erosion processes and has wide public acceptance. A major cost of revegetation is the addition of amendments, such as compost, to allow plant establishment. In this paper we explore whether arbuscular mycorrhizal fungi (AMF) can help support plant growth in tailings at a reduced compost concentration. A greenhouse experiment was performed to determine the effects of three AMF inocula on biomass, shoot accumulation of heavy metals, and changes in the rhizosphere microbial community structure of the native plant Prosopis juliflora (mesquite). Plants were grown in an acidic lead/zinc mine tailings amended with 10% (w/w) compost amendment, which is slightly sub-optimal for plant growth in these tailings. After two months, AMF-inoculated plants showed increased dry biomass and root length (p<0.05) and effective AMF colonization compared to controls grown in uninoculated compost-amended tailings. Mesquite shoot tissue lead and zinc concentrations did not exceed domestic animal toxicity limits regardless of whether AMF inoculation was used. The rhizosphere microbial community structure was assessed using denaturing gradient gel electrophoresis (DGGE) profiles of the small subunit RNA gene for bacteria and fungi. Canonical correspondence analysis (CCA) of DGGE profiles showed that the rhizosphere fungal community structure at the end of the experiment was significantly different from the community structure in the tailings, compost, and AMF inocula prior to planting. Further, CCA showed that AMF inoculation significantly influenced the development of both the fungal and bacterial rhizosphere community structures after two months. The changes observed in the rhizosphere microbial community structure may be either a direct effect of the AMF inocula, caused by changes in plant physiology induced by AMF, or a combination of both mechanisms.